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There are two drawbacks to frame-by-frame animation: First, it's labor intensive; second, it creates large files. Here Katherine Ulrich discusses the two ways Flash mitigates both problems with a process called tweening.

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THERE ARE TWO DRAWBACKS to frame-by-frame animation: First, it's labor intensive;
second, it creates large files. Flash offers a way to mitigate both problems
with a process called tweening.

To tween an object, Flash creates a series of incremental changes to
that object; these changes are simple enough so Flash can describe them all
mathematically. Flash does two types of tweening: motion
tweening and shape tweening.

Both types of tweening follow the same basic pattern. You give Flash the beginning
and end of the sequence by placing objects in keyframes. You tell Flash to spread
the change out over a certain number of steps by placing that number of in-between
frames between the keyframes. Flash creates a series of images with incremental
changes that accomplish the action in the desired number of frames.

Motion Tweening Versus Shape Tweening

The key to deciding whether to use motion tweening or shape tweening is to
ask yourself whether you could make this change via a dialog box or inspector
window. If the answer is yes, then Flash can do the change with motion tweening.
If the answer is no—if the change requires redrawing the shape of a vector
object—Flash must use shape tweening.

Another important distinction between motion tweening and shape tweening
is that motion tweening works only on groups and symbols. Shape tweening
works only on editable shapes. Sometimes you can arrive at the exact same
tweening effect with either a motion tween or a shape tween.

If you want to tween a multipart object—say, a robot constructed of
many separate objects—and you don't want to tween each object separately,
you'll need to make that object a group or symbol. Once it's a symbol,
you can only tween it with motion tweening. If you want to create morphing
effects—say, transforming a pumpkin into a magic coach—you must use
shape tweening. In addition, if you want Flash to move a tweened object
around the Stage along a curving path (as opposed to a straight line),
you must use motion tweening.